As legislators outlawed these drugs, manufacturers simply tweaked the formulas to get new, legal versions. Authorities realized they were always behind.

So federal lawmakers passed a bill to outlaw designer drugs in 1986. But it has been largely ineffective.

Called the Federal Analog Act, the law states that any chemical "substantially similar" to a controlled substance will be treated as one, if intended for human consumption.

It was the first time the government had tried to outlaw entire groups of drugs.

And it should have stopped bath salts.

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Over the past century, synthetic drugs have been born in laboratories, usually for medical purposes.

Bath salts are primarily derivatives of the drug cathinone. One of the original versions was first distributed as an antidepressant. But people started abusing these drugs. Authorities outlawed cathinone in 1993.

Then, in late 2010, authorities in Louisiana noticed a similar drug in packages labeled "bath salts" and "plant food," available in convenience stores and online.

They were legal — unless prosecutors used the Federal Analog Act.

The cathinone derivatives found in "bath salts"— methylone, MDPV, mephedrone — were structurally similar to cathinone, and they provided similar effects.

But prosecutors have shied away from using the very act designed to stop drugs like these.

In fact, the Federal Analog Act has never been used in the Middle District of Florida.

It is just too weak, several lawyers told the Tampa Bay Times.

One problem: It's hard to define "similar," said Florida Attorney General Pam Bondi.

These cases often turn into battles of experts — expensive cases to mount. The defense's witnesses describe the compounds' differences. The prosecution's witnesses talk about similarities.

Meanwhile, the law's vagueness poses another legal challenge, explained attorney Joseph Ruddy, chief of narcotics for the U.S. Attorney's Office in Tampa.

For a law to be used in court, the defendant must have known he did something illegal.

Analog Act cases have lost in court after the defense argues that the accused had no idea the drug was "substantially similar" to another, already outlawed one. It's complicated chemistry, and if expert witnesses cannot agree, how was the defendant to know?

"They can say they did not knowingly violate the law," Ruddy said.

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So, what can authorities do in the fight against designer drugs?

They can strengthen the Analog Act.

One lawyer laid out his suggestions in the University of Pennsylvania Law Review, saying that "substantially similar" needs to be further refined — perhaps by saying the molecule can only differ by "one functional group."

Or the U.S. Drug Enforcement Administration could publish what it considers potential analogs, wrote attorney Greg Kau.

Another strategy is to continue adding more drugs to the list of controlled substances. To fill the time gap while these bills become laws, authorities can use emergency orders, as Bondi did with several versions of bath salts.

In a recent interview with the Times, she hinted at a sweeping package her office plans to introduce in the 2013 legislative session that would help authorities fight synthetic drugs in the state.

She said lawmakers cannot outlaw entire classes of drugs because some have legitimate uses. But there are other strategies.

"We're looking at what we can legally do from a business perspective and from a regulatory perspective," she said. "Because good kids are taking these drugs."